1. Field of the Invention
This invention relates to a method of producing direct reduced iron with a gas obtained by coal gasification, and more particularly to a direct reduced iron producing method provided with steps of gasifying coal to a coal-derived gas containing a reducing gas, and reducing iron ore by the reducing gas to produce reduced iron.
2. Description of the Related Art
Production of reduced iron has been widespread because a plant for direct reduction of iron ore can be built at relatively low cost and easily operated. In addition, such a production can be economically practical even with a small-scaled plant. It is a common practice of using natural gas as a fuel (also as a reducing agent) for the production. Specifically, it has been a customary practice to reform natural gas into synthesis gas by H2O or CO2 so as to reduce iron ore by the synthesis gas.
Synthesis gas having substantially the same ingredients as the above synthesis gas can be produced by gasifying coal in a furnace designed for coal gasification. Synthesis gas produced by gasifying coal (hereinafter, referred to as xe2x80x9ccoal-derived gasxe2x80x9d) contains reducing gas consisting of CO and H2 as main ingredients, and CO2, H2O, H2S, etc. as sub ingredients.
Since the reduced iron production consumes a large volume of fuel, it is often the case that plants for reduction of iron ore are built close to fields of fuel (around gas fields). However, potential demands for producing reduced iron by using coal as a fuel (also as a reducing agent) instead of natural gas cannot be neglected especially in a region where inexpensive natural gas is hard to obtain but abundant coal is available. Particularly, producing direct reduced iron with use of coal-derived gas is regarded as a most practical technology because each process thereof is performed in a satisfactorily sophisticated manner. As a matter of fact, constructing parties and builders of plants for producing reduced iron researched processes of producing direct reduced iron with use of coal-derived gas. The results of their research were disclosed in T. A. Lepinski, M. R. Jones, Iron and Steel Engineer, Oct. 1982, pp. 23-28, P. E. Duarte, E. O. Gerstbrein, H. Smegal, Proceedings, AIC Conferences 3rd Annual Asian Steel Summit, 1997. This fact reveals that interest is increasing in this technical field.
However, a commercial plant aiming at producing direct reduced iron with use of coal-derived gas has not yet been put into practice. This is because building such a plant involves economical problems since invariable cost (fixed cost) such as construction cost for a coal gasification furnace and peripheral facilities is high. The fixed cost has not been successfully suppressed because each of the production processes are sophisticated and cannot be further simplified. Therefore, in order to lead this technology to a commercial success, required is an idea of suppressing variable cost by (a) reducing fuel cost due to improvement of heat efficiency or (b) utilizing inexpensive coal resources which has not been available in the conventional technology.
Considering (a) improvement of heat efficiency in producing direct reduced iron with use of coal-derived gas, the most important matter is how to utilize waste heat resulting from (A) coal-derived gas and (B) exhaust gas emitted from top part of a furnace for reducing iron ore (hereinafter, simply referred to as xe2x80x9ctop gasxe2x80x9d). Waste heat obtained from coal-derived gas (A) and top gas (B) each amounts to 200 to 400 kcal per kg of reduced iron (namely, in terms of calorie per 1 kg of reduced iron product, 200 to 400kcal=200xc3x974.18605 to 400xc3x974.18605 kJ=836 to 1672 kJ). The sum of waste heating value of coal-derived gas (A) and top gas (B) occupies about 20% with respect to the sum of the theoretical heating value requirement for producing the reduced iron and the waste heating value throughout the production processes.
It is desirable to perform hot feeding of coal-derived gas (feeding coal-derived gas to a predetermined facility such as a furnace for reducing iron ore at a sufficiently high temperature without being cooled to an atmospheric temperature) in order to most efficiently utilize waste heat obtained from coal-derived gas (A). On the other hand, taking into account an adverse affect that sulfur compounds such as H2S contained in coal-derived gas may impart to quality of resultant reduced iron product, it is desirable to desulfurize coal-derived gas. In view of these, it is desirable to perform hot desulfurization with respect to coal-derived gas (namely, desulfurizing coal-derived gas at a sufficiently high temperature suitable for desulfurization without cooling the gas to an atmospheric temperature).
As to the idea of how to utilize waste heat derived from top gas (B), it is required to fabricate a novel heat recovery system for the top gas (B) having relatively low pressure and temperature.
The following technology has been provided with respect to hot feeding and hot desulfurization of coal-derived gas (A). Specifically, U.S. Pat. No. 4,260,412 proposes an idea of obtaining coal-derived gas in a fluidized bed gasification furnace internally equipped with a desulfurizer and performing hot feeding of gas to an iron ore reducing furnace by way of a reheating furnace. U.S. Pat. No. 4,173,465 does not disclose a specific type of gasification furnace but suggests a process of hot desulfurizing coal-derived gas (desulfurization out of a furnace) on a movable bed of limestone. In any case, mixing coal-derived gas with top gas (B) which has been cleaned and cooled (hereinafter, referred to as xe2x80x9crecirculating gasxe2x80x9d) enables to lower the temperature of the coal-derived gas to a suitable level for hot desulfurization. This is conceived one of the effective and economical techniques of utilizing waste heat derived from coal-derived gas from the viewpoint of heat balance because sensible heat resulting from coal-derived gas is directly utilized in the process.
However, neither U.S. Pat. Nos. 4,260,412 nor 4,173,465 discloses the idea of utilizing top gas (B).
Top gas (B) has such a large fluid rate as 1.5 to 3Nm3 per kg of reduced iron product, but has a relatively low temperature and pressure (about 400xc2x0 C. and 2 bar), respectively. Specifically, since the temperature of the gas (B) is relatively low despite its large calorific capacity as a heat source, it is not easy to recover heat from the gas. Although the heat recovery can be attained by a heat exchange between the top gas and recirculating gas in order to meet heat balance in the process, the efficiency of such a heat exchange is considerably low due to low gas-to-gas heat transfer coefficient. Therefore, an expensive heat exchanger having a satisfactorily large heat transmission area is required. Even in the technical field of producing direct reduced iron with use of natural gas which has been primarily conducted nowadays, the heat recovery from top gas has been given up in most of the cases. However, there is a need of finding an effective heat recovery in the field of producing direct reduced iron with use of gas obtained by coal gasification. In this field, suppressing variable cost is a more significant task.
Regarding effective use of waste heat derived from coal-derived gas (A), both of U.S. Pat. Nos. 4,260,412 and 4,173,465 disclose direct cooling by mixing with recirculating gas to set the temperature of the coal-derived gas to a suitable temperature for hot desulfurization (400 to 900xc2x0 C.). This is one of the inexpensive and effective waste heat utilizing techniques as mentioned above. However, neither U.S. Pat. No. 4,260,412 nor 4,173,465 discloses effective measures for a case that a pressure in the gasification furnace is greater than that in the iron ore reducing furnace.
Reduced iron production plants currently under operation produce about 500,000 ton/year as a minimum unit on a commercial scale. In view of this, it is required to build a gasification furnace capable of producing coal-derived gas as much as 50,000 Nm3/h. Presumably, such a large-scaled gasification furnace is expected to have a pressure of 10 to 30 bar, which is exceedingly higher than that in the iron ore reducing furnace (about 2 bar). Such an exceedingly high pressure in the gasification furnace may involve the following problems.
In the case of a fluidized bed gasification furnace in which desulfurization is performed as disclosed in U.S. Pat. No. 4,260,412, it is necessary to draw vapor or part of recirculating gas into the fluidized bed gasification furnace to keep inside the furnace at a temperature around 800xc2x0 C. which is suitable for desulfurization. In the case where vapor is introduced, gas after the vapor introduction has a lower gaseous ratio of [(CO+H2)/(CO2+H2O)] than what is necessary in the iron ore reducing furnace. Accordingly, reduction power of the resultant gas is not sufficient, thereby obstructing direct feeding of such a gas having a lowered reduction power to the iron ore reducing furnace.
On the other hand, in the case where recirculating gas is introduced into the fluidized bed gasification furnace, a higher pressure in the gasification furnace requires introduction of recirculating gas in a pressurized state. This may increase compression power of recirculating gas. It should be noted that the aforementioned gaseous ratio of a make-up coal-derived gas (i.e., a coal-derived gas continuously supplied to the system) itself may be desirably set to 2 or more, although the ratio can be increased by mixing with recirculating gas in the fluidized bed gasification furnace.
In the case of performing desulfurization outside the furnace as disclosed in U.S. Pat. No. 4,173,465, the following drawback may occur. Specifically, it is necessary to mix recirculating gas in a pressurized state with coal-derived gas in order to set the temperature of the coal-derived gas at a suitable level for desulfurization. Namely, if a pressure in the gasification furnace is high, the pressure of the coal-derived gas is raised with the result that compression power of the recirculating gas is raised. In view of this, it is necessary to depressurize the coal-derived gas prior to mixing with recirculating gas to avoid such undesirable compression power rise. However, it is not easy to depressurize the coal-derived gas which has a high temperature. It is required to pre-cool the coal-derived gas to a temperature necessary for desulfurization, prior to depressurizing. Temperature control for such pre-cooling is conceived to occupy a large portion of temperature adjustment throughout the process. However, U.S. Pat. No. 4,173,645 is silent about measures for such a temperature adjustment as required for pre-cooling. There is proposed an idea of installing a waste heat boiler for pre-cooling. However, this does not provide an economical solution, nor does it provide a solution for improving heat efficiency.
Furthermore, neither U.S. Pat. No. 4,260,412 nor 4,173,465 explicitly recites pre-processing of coal. However, the following problems are involved concerning pre-processing of coal. In the case where coal has a low degree of carbonization and a high moisture (i.e., low-grade or B-grade coal), resultant gas obtained by gasification of low-grade coal has a low gaseous ratio. Even if hot desulfurization is performed in a desirable manner, reduction power of gas obtained from gasification of the low-grade coal is insufficient. It is difficult to perform direct hot feeding of such a gas to an iron ore reducing furnace. Accordingly, the low-grade coal cannot be used as it is. As mentioned above, developing a technology of utilizing low-grade coal which has a low carbonization and high moisture and therefore is available at an inexpensive cost is one of significant tasks in order to suppress variable cost due to (b) utilization of inexpensive coal resources, which has not been accomplished in the conventional technology. Low-grade coal is coal such as sub-bituminous coal, brown coal, and peat which have high moisture resulting from low degree of carbonization.
In view of the above problems residing in the prior art, an object of this invention is to provide a method of producing direct reduced iron with use of a coal-derived gas having an improved heat efficiency and improved economical effect.
More specifically, it is an object of this invention to provide a method of producing direct reduced iron with a coal-derived gas that enables to efficiently utilize waste heat derived from a top gas (B) as well as utilizing coal having a low degree of carbonization and a high moisture. It is still another object of this invention to provide a method of producing direct reduced iron with a coal-derived gas that enables to efficiently utilize waste heat derived from a coal-derived gas (A) even in a case where a pressure in a gasification furnace is higher than that in a furnace for reducing iron ore.
According to an aspect of this invention, a method of producing direct reduced iron with use of a coal-derived gas includes steps of heating coal to lower the moisture of the coal; gasifying the moisture-lowered coal in a coal gasification furnace to produce a coal-derived gas containing a reducing gas; and reducing iron ore by utilizing the reducing gas in an iron ore reducing furnace. In the step of heating coal, the coal is heated with use of an exhaust gas from the iron ore reducing furnace.
With this arrangement, the heat efficiency can be enhanced by utilizing the waste heat derived from the exhaust gas from the iron ore reducing furnace and by adjusting the moisture of coal to be gasified.
In the method, the coal-derived gas is desirably depressurized by a power recovery apparatus and the depressurized coal-derived gas is fed to the iron ore reducing furnace. This also can enhance the heat efficiency by utilizing the waste heat of the coal-derived gas.
It is also preferable that the exhaust gas from the iron ore reducing furnace that has been used for the lowering coal moisture is purified by removing non-reducing gas therefrom to obtain a recirculating gas and part of the recirculating gas is mixed with the depressurized coal-derived gas prior to being fed to the iron ore reducing furnace. Such a mixing is advantageous because the depressurized coal-derived gas can be cooled to a suitable temperature for the following process.
Moreover, the depressurized coal-derived gas is preferably subjected to hot desulfurization prior to being fed to the iron ore reducing furnace. More preferably, the mixed gas of the depressurized coal-derived gas and the recirculating gas is subjected to this desulfurization.
Part of the recirculating gas may be mixed with the coal-derived gas prior to being depressurized. This can lower the gas temperature to a suitable temperature for the depressurizing.
In the step of heating coal, the exhaust gas from the iron ore reducing furnace can be utilized directly or indirectly. The coal may be heated by a direct contact with the exhaust gas from the iron ore reducing furnace. Alternatively, the exhaust gas from the iron ore reducing furnace heats another gas due to a heat exchange therebetween and, in the step of heating coal, the coal is heated by a contact with the heated another gas. It is also possible that: the exhaust gas from the iron ore reducing furnace is purified by removing non-reducing gas therefrom to obtain a recirculating gas, part of the recirculating gas is used as a fuel in a reheating furnace for reheating of the coal-derived gas so as to feed the reheated coal-derived gas to the iron ore reducing furnace and, in the step of heating coal, the coal is heated by a contact with a gas exhausted from the reheating furnace.
According to another aspect of this invention, a method of producing direct reduced iron with use of a coal-derived gas includes steps of gasifying coal in a coal gasification furnace to produce a coal-derived gas containing a reducing gas; depressurizing the coal-derived gas by a power recovery apparatus; and reducing iron ore by utilizing the reducing gas in the depressurized coal-derived gas in an iron ore reducing furnace.
According to a further aspect of this invention, a system for producing direct reduced iron with use of a coal-derived gas includes a moisture adjuster which lowers the moisture of coal; a coal gasification furnace which gasifies the moisture-lowered coal to produce a coal-derived gas containing a reducing gas; an iron ore reducing which reduces iron ore by utilizing the reducing gas; and an exhaust gas utilizer which utilizes an exhaust gas from the iron ore reducing furnace for lowering the coal moisture.
It is preferred that the system further includes a power recovery apparatus provided between the coal gasification furnace and the iron ore reducing furnace for depressurizing the coal-derived gas. The power recovery apparatus desirably includes an expansion turbine.
The exhaust gas utilizer may include a gas passage which allows the exhaust gas to flow into the moisture adjuster from the iron ore reducing furnace. Alternatively, it may include a heat exchanger which transfers the heat of the exhaust gas to another gas and a gas passage which allows the another gas to flow into the moisture adjuster from the heat exchanger. In these cases, the system preferably has a gas cleaner which purifies the exhaust gas that has been utilized by the exhaust gas utilizer; a mixer which mixes the coal-derived gas depressurized by the power recovery apparatus with part of the cleaned gas; a desulfurizer which desulfurizes the mixed gas; a reheating furnace which heats the desulfurized gas; and a gas supplier which supplies the heated gas into the iron ore reducing furnace.
It is also possible that the system has a gas cleaner which purifies the exhaust gas from the iron ore furnace; a mixer which mixes the coal-derived gas depressurized by the power recovery apparatus with part of the cleaned gas; a desulfurizer which desulfurizes the mixed gas; a reheating furnace with a burner, in which the de-sulfurized gas is heated by the burner; and a gas supplier which supplies the heated gas into the iron ore reducing furnace. In this case, the exhaust gas utilizer may include a first gas piping which allows another part of the cleaned gas to flow from the gas cleaner into the burner of the reheating furnace so as to use the another part of cleaned gas as a fuel; and a second gas piping which allows a gas exhausted from the reheating furnace to flow from the reheating furnace into the moisture adjuster.
Moreover, the system can have a gas cleaner for purifying the exhaust gas from the iron ore furnace that has been utilized by the exhaust gas utilizer; a mixer which mixes the coal-derived gas produced in the coal gasification furnace with part of the cleaned gas; and a gas supplier which supplies the mixed gas into the power recovery apparatus.
According to a still further aspect of the present invention, a system for producing direct reduced iron with use of a coal-derived gas includes a coal gasification furnace which gasifies coal to produce a coal-derived gas containing a reducing gas; a power recovery apparatus which depressurizes the coal-derived gas; and an iron ore reducing furnace which reduces iron ore by utilizing the depressurized gas.
These and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawing.